1. Field
The disclosed embodiments fall in the domain of an aircraft that must land and take off on relatively short airstrips.
In particular, the disclosed embodiments involve civil transportation aircraft for which the combined arrangement of the fuselage, of the various aerodynamic surfaces and of the propulsion engines permits making an aircraft capable of operating on terrains of limited length and complying with the certification requirements of civil aircraft in particular in cases of takeoff and approach with failure of one engine.
The aircraft according the disclosed embodiments also permits making takeoffs and approaches with reduced noise impacts on the ground which is essential when using the aircraft in concentrated population environments such as mountainous environments or islands.
2. Brief Description of Related Developments
To reduce the takeoff and landing distances of aircraft for which the lift is ensured by a wing or by a set of lift aerodynamic surfaces, the solution that is the most generally used is to implement means that permit reducing the minimum flight speeds.
Indeed, reducing the minimum flight speeds enables an aircraft to limit the acceleration distance on the ground during takeoff and the deceleration distance on the ground during landing.
Increasing the thrust provided by the propulsion engines, in particular the ratio between engine thrust and aircraft weight, is also a practical solution to diminish the takeoff distances through a much stronger acceleration to reach the takeoff speed but the increase of thrust rapidly finds economic limits, in particular in the area of civil transportation aircraft, and causes sound nuisances that today are no longer acceptable in airport environments.
A first, widely used technique to increase the lift of the aircraft consists in arranging onto the lifting surfaces, in particular the wings, high-lift devices that result in pushing back the takeoff phenomena at lift factor values that are higher than those possible with a lifting surface not provided with such high-lift devices.
The trailing edge high-lift devices or wing flaps, also called flaps, and leading edge high-lift devices or leading edge slats, are widely used on civil transportation aircraft.
As such, for a conventional civil aircraft for which the wing has a lift factor around 1.6 without high-lift device (a so-called smooth configuration), in practice, lift coefficients between 2.5 and 3 are obtained when the high-lift devices are deployed for landing configurations.
In practice, the increase obtained for the maximum lift of an aircraft wing depends on the complexity of the high-lift systems and of the extension of the chord and the span of said systems.
Numerous high-lift forms, single, double or triple flaps, with or without slot, are known but on the one hand, it is difficult in practice to obtain lift factors of more than 3.5 and on the other hand, obtaining a high-lift coefficient is accompanied by complex high-lift systems that are as such heavy, expensive to make and expensive to maintain.
Finally, complex high-lift devices are the sources of parasitic drag that deteriorates the smoothness of the aircraft flight during the cruising phase, for a smooth configuration.
The solution of the conventional high-lift devices is as such inadequate to solve the problems encountered by a civil transportation aircraft that has to operate from short airstrips in urban environments and to keep cruising performances in accordance with the expectations for a line aircraft.
A second technique, associated with the first, to increase the lift of a wing is known by the designation flap blowing or upper surface blowing of the aerofoils.
By accelerating the air flow over the aerodynamic surfaces, this technique permits obtaining lifts that are higher than those associated with the simple progress speed of the aircraft in the air mass. Numerous variants of blowing exist.
For propeller aircraft for which the engines are secured to the wing, this technique is relatively practical to implement because naturally, the slipstream generated by the propellers involves the wing and the trailing edge flaps. On the Breguet 941 transport aircraft, this phenomenon has been used to obtain remarkable takeoff performances.
For jet engine aircraft, the techniques used or imagined consist in using the flows of the jet engines or an air flow taken from the jet engines to produce flap blowing (as on the Mc Donnell Douglas C17 cargo plane or on an experimental YC15 aircraft by the same builder) or of the upper surface of the wing (as on the NASA QSRA experimental aircraft).
However, these blowing techniques, if they permit looking for very increased lift coefficients up to a lift coefficient of 8 or more, are not without fault.
A first problem is linked to the fact that the motor installations must be designed for an efficient blowing in the low speed configurations of the aircraft; the effect of this is to penalize the cruising conditions during which these devices are useless, in particular by increasing the mass and the drag of the aircraft and by degrading the propulsion efficiency.
A second problem is linked to the extreme conditions in which operate the complex high-lift devices that, subject to the jet engine blast, undergo accelerated structural aging.
A third problem involves the handling of the aircraft in case of loosing an engine during a take-off or approach phase.
In this case, the risk of lift dissymmetry between the side of the wing carrying the failed engine and the other side of the wing generates a roll torque couple of rolls that risks causing the full loss of command over the aircraft. To limit the consequences of an engine loss, the known solutions make engine couplings, for instance by using a coupling arm for the propellers on the Breguet 941, or solutions looking to restore a balance of the lifts on each side of the wing by modifying the flap configurations, solutions that in general are complex. Not only are such solutions costly but also, the consequences of a failed engine that would result in losing control over the aircraft make such devices very difficult to certify on civil transportation aircraft, which partially explains that only experimental aircraft or military aircraft have implemented such solutions.
A third technique to increase the lift of an aircraft consists in using a portion of the engine lift to compensate a portion of the aircraft weight. According to this method called vector thrust, the jets of the engines on the aircraft are inclined downward, in general deviated in such a way that a component of the engine thrust is directed downward. The military Harrier aircraft by Hawker Siddley Aviation is a case of successful application of the vector thrust that even permits this jet engine to take off and to put itself down vertically at the expense of a thrust/weight ratio of more than 1 which is economically unthinkable for a civil transport aircraft.
As such, it appears that none of the known solutions permits achieving high-lift coefficients, higher than those obtained on the present civil transport aircraft, necessary for an aircraft with short takeoffs and landings under acceptable conditions for a civil transport aircraft in terms of penalties of cruising performances, mass and costs, and of certification.